Microphone and microphone granules



Dec. 14, 1954 w. o. BAKER ETAL l 2,697,135

MICROPHONE AND MICROPHONE GRANULES Filed April 28, 1951 F'LU/D/TV NGE/VT OFANGLE OF REFUSE) (com m R m l, POLV QUARTZ 0. 9A/(ER 0. GR/SDAL /Nl/E/VTORS ATTORNEY 2,697,136 Patented Dec, 14, 1954 2,697,136 MICROPHONE AND MICROPHONE GRANULES William O. Baker, Morristown, and Richard O. Grisdale, Short Hills, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 28, 1951, Serial No. 223,635 14 Claims. (Cl. 179-122) This invention relates to microphones and similar devices i which changes in mechanical pressure oii a granular mass are converted into changes in the electrical resistance of faces a carbon coating pyrolytically deposited from a hydrocarbon gas (United States Patent 1,973,703 to F. S. Goucher and C. J. Christensen and United States Patent pared with anthracite.

e devices of the present invention embody a granular material possessing one or more of several advantages adapted to the particular requirements of the circuit in which the microphone is to be used.

referred to above as well as the particle size and particle A' This controllable density, together with the controllable iiuidity and controllable resistivity to be described below, considerably widen design potentialities for microphones in which polymer 2, carbon particles are used. The polymer carbon particles be readily formed in perfect spherical shape in any la wide range without a substantial effect upon the moduation.

The desirable modulation eciency of polymer carbon surfaces can be utilized in microphone granules of other structure as, for instance, in granules formed of rounded particles of refractory non-conductive materials such as fused quartz or puffed silica gel coated with a layer of polymer carbon.

full benet of their superior modulation is n ot utilized.

bases for the deposition of a layer of carbon pyrolytically deposited from a hydrocarbon gas.. Although the moduhe present invention includes all of the microphone granule structures s'et forth above. The manner in which these granules can be utilized in a microphonie. device ig. 2 is an' enlarged View in section of `a portion of the microphone shown in Fig. l;

Fig. 3 is a chart showing the relative modulation eciencies of various microphone materials and the variation of these eiciencies with variation in the uidity of the materials;

Fig. 4 is a sectional view of a microphone granule consisting of a polymer carbon sphere;

Fig. 5 is a diagrammatic sectional view of a microphone granule consisting of a polymer carbon sphere coated on its surface with carbon pyrolytically deposited from a hydrocarbon gas;

Fig. 6 is a diagrammatic sectional view of a microphone granule consisting of a quartz base with a surface layer of polymer carbon; and

ig. 7 is a diagrammatic sectional view of a microphone granule consisting of a quartz base covered with a layer of polymer carbon and having an external layer of be discussed in more detail below, a microphone having a sealed granule chamber such as shown in Figs. 1 and 2 is particularly desirable for use with granules of this type.

The microphone shown in Figs. 1 and 2 is made up of a case 1 which carries, by means of support members 2 and 3, a dished and corrugated diaphragm 4 At the center of the diaphragm 4, a spherical container 5 is mounted as by means of solder or adhesive.

The spherical container is made up of an upper metal hemisphere 6 and a lower metal hemisphere 7, each having a projecting lip 8, 9. Between the projecting lips is mounted an inertia diaphragm 10 formed of metal and carrying at itscenter an inertia and contact member 11, which is also formed of metal and which is in electrical contact with the diaphragm 10. The chamber defined by the lower hemisphere 7 and the diaphragm 10 is filled with a mass 12 of the granular microphone material of the present invention. The diaphragm i is electrically insulated from the microphonic material 12 and from the lower hemisphere 7 by means of an insulating coating 13 on its lower surface which may be formed of a thin layer of enamel, either organic or vitreous. An air passage is formed in the contact member 11 to equalize the air pressure in the upper and lower chambers of the sphere when the diaphragm vibrates.

The shpere S is preferably filled with a suitable inert gas, such as dry helium or nitrogen, to prolong the life of the polymer carbon microphonic material. When. the sphere is lled with a special atmosphere, it is necessary that the two hemispheres 6 and 7 be sealed adequately where the lips 8, 9 meet the diaphragm 10 to prevent escape of the gas and entry of the air from the atmosphere. The metal surface of the lip S can be hermetically sealed to the upper metal surface of the diaphragm 10 by soldering, brazing or welding or by means of an adhesive, proper steps being taken to insure electrical contact between the diaphragm and the hemisphere.

The insulated lower surface of the diaphragm 10 may be sealed to the metal surface of the lip 9 by means of an adhesive or a glass-to-metal seal in such manner as to avoid electrical contact between the diaphragm and the lower hemisphere. The lower hemisphere 7, which is in electrical contact with the mass 12 of microphonic material, is electrically connected, by means of a wire 15 to a terminal 16.

In the operation of the microphone, sound waves impinging on the diaphragm 4 cause it to vibrate and with it the sphere 5. The inertia of the member 1l thereupon creates a relative motion between the diaphragm 10 and the sphere 5, causing a periodic change in the pressure of the diaphragm l0 upon the microphonic mass 12 The resulting change in the electrical resistance of the mass 12 is utilized by the passage of an electric current from the lower hemispherethrough the mass 12, through the diaphragm 10 and iinally through the upper hernisphere 5 and diaphragm 4 to the microphone case. The structure and operation of a microphone of this type are more particularly described arid claimed in the copending application of H. Eckardt, Serial No. 97,972,

tiled June 9, 1949, now Patent No. 2,567,368.

The chart in Fig. 3 showsthe relative modulation, under certain conditions of test, of microphone granules of the present invention and certain prior known These tests were carried out ina test cell of essentially cylindrical shape with a vibratile diaphragm mounted as one end of the cylinder. cell was filled with a microphonic material and the diaphragm was vibrated at a frequency of 1 kilocycle the contact member 11, through f2 per second and with an amplitude of several hundred angstroms. The modulation was measured as the ratio of the instantaneous change in resistance of the mass on vibration to the average agitated resistance of the mass.

It can be seen from the chart that anthracite microphone granules measured under these conditions gave a modulation of about l3 per cent. Polymer carbon particles having a particle size corresponding to to mesh (passing a No. 50 sieve having a sieve opening of 0.30 millimeter but not passing a No. 60 sieve having a sieve opening of 0.25 millimeter) gave a modulation varying between 33 per cent and 38 per cent, depending upon the fluidity of the mass of particles. Polymer carbon particles having a particle size of 60 to mesh (passing a No. 60 sieve but not passing a a No. 80 sieve having a sieve opening of 0.177 millimeter) gave a modulation of between 2l per cent and 3l per cent depending upon the fluidity. The anthracite particles tested also had a size of 60 to 80 mesh.

The iluidity of the mass of particles is given on the chart in terms of the cotangent of the angle of repose of the mass. The cotangent of the angle of repose 'of a mass consisting almost entirely of substantially perfect spheres of polymer carbon of 60 to 8O mesh was measured as about 2.15. The uidity of the mass can be decreased by adding varying amounts of irregular particles formed by the agglomeration of spheres as will be discussed in more detail below. By the addition of large amounts of agglomerates, the fluidity was reduced to a value ofabout 1.3 which was the value measured The agglomerates which at of the that they for the anthracite particles. were added had a particle size the s ame as th spheres to which they were added, indicating were agglomerates of spheres of a smaller size:

The chart shows that polymer carbon particles having a coating of carbon deposited pyrolytically from a hydrocarbon gas gave a modulation, at various values or" iluidity, which was somewhat lower than that of uncoated polymer carbon of similar particle size lbut substantially higher than that of quartz base particles coated in the same manner with pyrolytic carbon. Values of modulation obtained for quartz particles coated first with polymer carbon and then with pyrolytic carbon (not shown on the chart) are substantially'the same as those obtained for polymer carbon particles coated with pyrolytic carbon.

The pyrolytic carbon was deposited upon the particles referred to above by heating them in a furnace rotating at 24 revolutions per minute at a temperature of ll50 C. for two hours while passing nitrogen containing 30 per cent methane through the furnace according to the method described in United States Patent 1,973,703 referred to above.

The chart in Fig. 3 shows that, for the microphonic materials of the present invention in the test cell described, the modulation tends to increase substantially as the fluidity decreases. This indicates that the particle surfaces have an intrinsically high modulation efficiency but that some of this modulation is lost due to slippage of the particles past one another as they are subjected to varying pressure by the diaphragm. This slippage is dependent upon by the shape of the chamber in which the particles are contained and varies with variations in the microphone structure.

By reducing the fluidity of the particles, the slippage is reduced and, since the inherently high modulation efficiency of the surface (due to its microscopic smoothness and its sphericity at the points of Contact) is not unduly altered, tain microphone structures, optimum values of fluidity are found, with the modulation higher and lower uidities. It is an advantage of the polymer carbon base materials of the present invention that their iluidities can be adjusted to optimum valucs very simply by the proper proportioning of spheres and agglonierates.

The manner in which spheres and agglomerates of polymer carbon can be produced is more particularly described and claimed in the copending applications of W. O. Baker and R. O. Grisdale Serial No. 223,633, W. O. Baker and F. H. WinslowSerial No. 223,638, and W. O. Baker, R. O. Grisdale and F. H. Winslow Serial No. 223,634, all filed on the saine day as the present application.

As indicated above, polymer carbon consist substantially entirely of carbon, containing a are formed by the pyrolytic dehydrogenation of similarly shaped bodies of hydrocarbon polymers or of polymers containing carbon, hydrogen and a minor amount of the additional element. The carbon or modified carbon which is produced is harder and less graphitic than any other carbon hitherto reported in the literature other than diamond.

Since the microphonic particles of the present invention which are the most readily produced are those which are spheres or agglomerated spheres formed substantially entirely of carbon and which are produced by the pyrolytic dehydrogenation of hydrocarbons, the roduction of this type of material will be described rst.

Any solid hydrocarbon polymer body which can be converted in situ to a carbon body by pyrolysis will yield a product having the desirable microphonic properties referred to above. Any hydrocarbon polymer can be so converted if it has an adequate degree of crosslinking. A hydrocarbon polymer which has no substantial cross-linking, such as polystyrene, will be substantially completely converted to volatile products upon pyrolysis and will leave no carbon residue.

If the cross-linking in the polymer body is suficiently great, as when the polymer is formed of trivinyl benzene, the body may be directly subjected to pyrolysis by heating in a nonoxidizing atmosphere rand will yield a solid, coherent, essentially carbon body of the bodies which or of carbon minor amount of an additional element,

same shape which contains anywhere from A30 per cent tionality of that monomer and adding-the products thus to 60 per cent or more of the carbon -originally presobtained. ent in the polymer. Polymersformed from monomers having an average ore often, however, the polymer bodies will be of such a nature, as when formed of divinyl benzene polymers, baking, be made to give high carbon yields upon pyrolysis. that they are insufliciently cross-linked to give such a high yield of carbon upon pyrolysis. When such bodies are heated in a non-oxidizing atmosphere, they may leave no residue at all or, at best, they may leave carbon amounting to no more than about l0 per cent of the original l0 nary air-baking. The polymers formed from the monoweight of the polymer. lf there is a carbon residue in spheres of this type. Such hollow spheres Will-obviously choice of individual monomers of the required functionave a much lower overall density than solid spheres of ality or by the proper proportioning of two or more monomers of different functionalities.

Spheres of a higher density are more often required a functionality of 4, trivinyl benzene Witha functionality since the most eicient transfer of mechanical energy of 6, divinyl naphthalene with a functionality of 4, vinyl from the diaphragm of the microphone to the mass of acetylene with a functionality of 6, divinyl acetylene wit carbon particles is obtained when the density of the aga functionality of 8, bis (p-vinyl phenyl) methane with a gregate of the carbon particles is properly matched to the functionality of 4, or vinyl butadienyl acetylene withial effective inertia ofthe diaphragm. Therefore, in order to functionality of 10. Similarly, naturally occurring -unsecure a higher density, an increased yield of carbon is saturated short-chain hydrocarbon polymers, such as lycopene or beta carotene (C40 Has) which contain eleven conjugated double bonds in their molecules, may be subjected to further polymerization to produce highly crossa maximum at between 200 C. and 300 C. and prefer- 00 linked materials for conversion to polymer carbon. ably at about 250 C. Although it is possible to obtain a small but substantial increase in the yield of carbon by other or with bifunctional hydrocarbon monomers may bel air-baking at the maximum temperature for as little as COPOIYHleU'Zed S0 as O Produce e Polymer 0f the desired two ours, more substantial increases in yield are obtained functionality. if the baking is continued for at least four hours For the Polymerization of the monomers or monomer mixtures greatest increase in carbon yield, the baking is continued can be accomplished in the conventional manner with or onger periods of, for instance, twenty-four hours or either the oriffinal monomers or the partially polymerized one wee or even two weeks VPreferably the polymer material,l while still plastic, being formed in the desired bodies are brought gradually from ,room temperature to l shape hns polymerization is conveniently accomplished t e maximum temperature while in contact with air. 40 by adding i per Cent by weight of benzoyl peroxide to the By this air-baking procedure, -Solid Carbon bodies .in material to be polytnei'ized and then heating it to a temyields of per cent or more ofthe weight ofthe polymer per-amro at which polymerization Occurs at a practical rate', as forv instance at temperatures between and hollow bodies with carbon yields of only 6 or 7 per cent. The air-baking can also be used to increase the carbon yields of highly cross-linked polymers, such as polymers of l 'mes its original volume in a having no substantial heat of solution.

trivinyl benzene, although the proportional increase in .As 1s IiP-0WD H the aft, POIYmelZaOH may be aecomyleld is less for those polymers which give a high yield i, plished with larger or smaller amounts of benzoyl peroxide without prior air-baking. A similar increase in yield of 50 as, fOr Instance, between 0-5 Pel' Cellt and Per Cem carbon can be obtained by a preliminary baking in cer- Othel' Polymerization Catalysis such es Cumefle hydro-- peroxide, t-butyl hydroperoxide, l-hydroxy cyclohexyl hydroperoxide-l, lauryl peroxide, stcaryl peroxide or other be controlled in two ways. First, the composition of the polymer can be chosen to give varying degrees of crossble-butyfonlflle meY be Used Wlfh the PGIYIIleFZeOH linking. Second, in polymers having intermediate degrees ,.v), eefelys, 1f desired, 1H any Sultable amount as, for iiiot cross linking, the density can be controlled by varying m Stance, beWeel 05 Per Cent and 0-2 Per Cent b/ Weight the amount of preliminary air-baking. 0f the POIYmeYIZebIe material- As indicated above, even with preliminary air-baking, a Where Polymer Carbon spheres al' e t0 be Produced, The certain minimum amount of cross-linking is required in Polymers formed Pl'edommamly from m0n0mef5`made m up of aromatic nuclei having unsaturated aliphatic hydro- The Cros-s lnkng of m carbon substituents thereon are particularly desirable since which essentially perfect spheres of small, controllable size iial volume in a thermodynamically inert solvent (having can be Produced. m hlgh yleld' The polyny.I ,ammatic ln the formation of polymer spheres by this method the material to be polymeri'zed is agitated, as by rapid stirring, together with a body of a non-solvent suspenl sion liquid, such as water. Under the inuence of they RUHCUOHSIY ef 2 and each eeeylenle triple bond C011' continuing agitation, the material to be polymerized tributes a functionality o f 4. The average functionality breaks up im@ Spherical globules dispersed in the sus. of a monomer mixture is computed by multiplying the pension liquid. The entire system is maintained at amol fraction of each monomer in the mixture by the funct 5 polymerizing temperature until. rigid, non-tacky polymer I polymerization in suspension can be continued until the requisite degree of crosslinking, as set forth above, has been achieved or the polymer spheres can be removed from the suspension after they have become rigid and non-tacky and .can be subsequently heated to complete their polymerization; The manner in which a partial yield of polymer spheres. this method is nown. by which high yields of sphres. is

spheres are produced. The

to the art. falling within a described and claimed in the F. H. Winslow, Serial N 1950.

According to this procedure, a liquid mass of material, to be polymerized, which contains a polymerization is rapidly stirred by a rotary stirrer into susas much by volume, and preferably times as much by volume, or water or an aqueous solution of an inorganic salt, they pH of which is maintained at a value 3 and and which has dissolved in it between 0.25 per cent and. 5 per cent, and preferably about 2 per cent, of a suspension stabilizing agent comprising polyvinyl alcoholv having a degree of hydrolysis of at least 95 per centand. preferably at least 98 per cent and having an intrinsic viscosity in aqueous solution of between 0.3 and 09 The temperature of the system is maintained between about 60 C. and 100 C., and preferably between about. 75 C. and 85 C., until the suspended spheres avc: polymerized to a rigid, non-tacky state. a

In this process, an increase in the rate of agitation and an increase in the concentration of the polyvinyl alcohol in the aqueous suspension medium tend to decrease the size of the spherical polymer particles which are produced. Similarly, the use of polyvinyl alcohols of decreasing degrees of hydrolysis or of increasing in-. trinsic viscosities tends to decrease the size of the spheres. With polyvinyl alcohols having degrees of hydrolysis and intrinsic viscosities falling within the range set forth above, a high yield of unagglornerated spheres, the greater proportion of which have a diameter falling within a. narrow range of size distribution, can be obtained withj` average diameters lying between .05 millimeter and millimeters. -Larger spheres can be obtained in lower yield by decreased agitation and lower concentrations ofpolyyinyl alcohol, particularly when the lower viscosity grades of polyvinyl alcohol are used. When it is desired to produce spheres of smaller diameter, down to .005 millimeter for instance, a polyvinyl alcoh l of lower degree of hydrolysis, for instance about a higher intrinsic viscosity, for instance about .0, may be used.

The fraction of polymerized material which is removed from suspension as agglomerated spheres, instead of individual spheres, can be converted to polymer carbon and used for forming microphone granules of reduced uidity as described above. The conversion of the agglomerated spheres to polymer carbon can be accomplished while they are still mixed with the polymer spheres or they can be separated, as on a vibrating table separator, prior to conversion.

The pyrolytic dehydrogenation of the spheres or agglomerated spheres to form the polymer carbon granules is carried out, with or without the preliminary baking in air. by heating in a non-oxidizing atmosphere to prevent loss of carbon to preserve the intrinsically high modulation eiciency of the surfaces. Nitrogen and helium are e maintained during pyrolysis, particularly during the latter stages. It is essential that the atmosphere be maintained as free as possible from traces of oxygen and Water vapor since these substances strongly affect the microphonic properties of the resulting carbon, particularly during the nal phase of pyrolysis at temperatures C. to l200 C. Where the gas to be used contains these substances they can be conveniently removed by adding a small amount (of the order of l0 per cent) of hydrogen and passing the mixture of gases rst over a palladium catalyst to convert the oxygen to water vapor and then through a drying tower filled with granular calcium hydride.

It has been found that the effect of the presence of oxygen or water vapor is to cause the formation of a thin surface layer of increased conductivity on the surfaceo the carbon particles. This layer of increased sri conductivity increases the effective area of electrical contact between adjacent particles and thus reduces the resistance of the mass of the particles although the overall resistivity of each particle has not been substantially changed. Since the effective area of electrical contact W of mechanical contact, the change of contact resistance with increased area of mechanical contact is reduced and the modulation is correspondingly reduced.

Much the same type of surface change appears to take place, at a considerably slower rate, when the polymer carbon granules are aged in contact with air at room temperature. As indicated above, this decrease in modulation efficiency upon aging can b e avoided by sealing the granules in an inert gas.

The pressure of the gas in the pyrolytic furnace is preferably maintained slightly above atmospheric to prevent seepage of air into the furnace. Any conventional type of controlled atmosphere furnace may be used. The non-oxidizing gas is continuously passed through the furnace in order to sweep away the gaseous products of pyrolysis. Preferably the particles are not tumbled during pyrolysis so as to avoid as far as possible any contact of the particle surfaces with contaminating gases.

The polymer bodies in the pyrolytic furnace are brought gradually .to the maximum temperature of pyrolysis so as to allow the gradual release of the gases which are developed and thus prevent destruction of the bodies. lt has been found that a temperature rise of about 200 C. per hour between about 300 C. and the maximum temperature yields desirable results. Obviously the bodies may be heated more slowly if desired, as for instance at an average rate of about 5 C. per hour. A more rapid rate of heating. up to about 500 C. per hour, may also be used. The temperature increase is preferably made continuous.

The residual amount of hydrogen remaining in the final carbon product is dependent upon the maximum temperature to which the bodies are brought during pyrolysis for a substantial period of time. A product consisting of at least 99 per cent carbon can be oroduced by carrying the pyrolytic temperature to 850 C. and maintaining the material at this temperature for one-half hour or more.

in a typical product subiected to pyrolysis at a ternperature increasing at the rate of 200 C. per hour until a temperature of 900 C. was reached and maintained at that temperature for one-half-hour, the hydrogen content was found to be 0.64 per cent by weight. After being maintained at 1000 C. for one hour. the hydrogen content was reduced to 0.36 per cent. The hydrogen content was reduced further to 0.23 per cent by heating one hour at ll00 C., to 0.12 per cent by heating one hour at l200 C. and to between 0.02 per cent and 0.01 per cent by heating one to three hours at l300 C. These values represent a hydrogen content of one hydrogen atom per twenty-three carbon atoms in the product heated to l000 C. and one hydrogen atom per four hundred to eight hundred carbon atoms in the product heated to l300 C.

The electrical resistivity of the product at 25 C. varies between about ohm-centimeters for a hydrogen content of about l per cent and about 10'2 ohm-centimeters (about three hundred times the resistance of graphite) for a hydrogen content not exceeding about .02 per cent.

When the resulting polymer carbon granules are to be used in a microphone, such as is shown in Figs. l and 2, in which they are sealed in an inert gas, it is preferable that they be kept out of contact with air or other contaminating gases between the time pyrolysis is completed and the time they are sealed in the microphone. When the ,granules are to be sealed in helium. `nitrogen. or other protective gas, it is desirable that the last portion of the pyrolysis. or at least the cooling of the granules from the maximum pyrolytic temperature. be carried out in helium or other protective gas and that the granules be maintained in an atmosphere of helium or other protective gas until they are sealed in the microphone.

Tf the granules become exposed to air` hydrogen or other deteriorative gas. they can be restored fn .almost their original characteristics by heating to l000 C. der a high vacuum or in an atmosphere of the protective gas until the bulk of the adsorbed deteriorative gas has been removed.

t 2,697,186 l 10 When it is desired to form granules in which polymer The desirable properties of polymer carbon for microcarbon formed as described above is coated with pyrolyphonic purposes are retained and-are sometimes enhanced tic car on, as s own in Fig 5, t is coating can be a 4 even when a minor proportion of the carbon 20 per cent plied y any or the well-known procedures for deposit or less, is replaced with another unitormly distributed ing such a coating from a hydrocar on gas, such as the 5 normally solid elementl T e introduction of these addiprocedure described above in connection with the prepational elements provides an additional advantage Since ration of the coated granules tested to give the data shown they permit the electrical resistivity of the product to in Fig. 3. `e controlled over a wi er range than is possible with As stated above the valuable modulation pioperties unmodified carbon. is additional element can be inof polymer carbon surfaces can be obtained, without the rodueed inofhe Polymer Carbon by forming llecafbon benefits of controllable density and tiuidity, by forming i'rorn a modied hydrocarbon polymer containing the a granule structure as shown in Fig 6 in which a coating eddluonol element, 1n n e requlred proportion IO the 0 p0 ymer carbon is formed 0n a granular base 0f any Carbon, 111 either the netWOIk Of [he pOlymel 0r 111 Slde refractory material, such as quartz groups or chains. As examples of elements which ma coating, on the base, of any of the polymers referred to boron, PbOSPboruS, Silver, titanium, lalufnlnuin, gefinaabove and then subjecting the coating to pyrolysis with Inuni, b1Sn1ul1, un, or other ineialS 01 niealloidS- By This carbon coating can be formed by first forming a la e introduced in this manner may be mentioned silicon,

or without preliminary air-baking, under the same condithe incorporation 0f Varying amounfS 0f Ille additional tions as set forth above. The same precautions as deelement, ille feSiSinCe of the resulting Polymer Carbon scribed above are necessary to preserve the stability of the bodies C2111 be adluSed to the mOSt desirable Values for polymer carbon surface against aging. any Parlieular microphone use.

he polymer lins to be converted to carbon films can As an example of the manner in which a polymer carbon be deposited in any convenient manner. Thus any of Containing silicon can be formed from a polymer conthe liquid monomers or onomer mixtures, containing taining Silicon in ille neiWolk Structure, Such a Polyiner a polymerization catalyst, can becoated on the surface Cnn be formed by the Polymerization 0f a Polyellyl Silane and then maintained at a polymerizing temperature until such as tetraallyl silane, methyl triallyl silane or dimethyl a pglymer 0f the required decree of Cr053-11nk1ng has dlaly'l Sllalle either 310116 OI OgellleI Wllll aIlOlheI' pOlybeen produced, Similarly, the monomer 0r a partially inerizable monomer such as divinyl benzene or trivinyl polymerized material which is still soluble may be disbenZene- Tile deSired Silicon i0 Cef bon ratio Can be obsolved in a volatile solvent, the solution may be coated tained by tbe propel Proportioning 0f Silicon-Containing on the surface, the solvent may be allowed to evaporate and hydrocarbon monomers or of two or more monomers and then the monomer or partially polymeri'zed material Containing different Silicon t0 Carbon ratios. may be further polymerized. Polymers containing silicon in side groups can be formed As indicated above, even when the polymer carbon by the copolymerization of silylstyrenes, such as trimethyl sur ace on a ierractory base is coated with pyrolytic car silyl styrene, triethyl silyl styrene or other trialkyl silyl on to produce a granule as shown in Fig. 7, the modustyrenes with a sulicient proportion of a polymerizable lation of a mass of the granules is still superior to that material of higher functionality to give the required deof a mass of granules of similar refractory material havgree of cross-linking, such as 20 per cent or more by ing a single layer of pyrolytic carbon of good microweight of divinyl benzene, trivinyl benzene, tetraalkyl phonic properties coated directly on its surface without 40 silane or methyl triallyl silane. the intermediate polymer carbon. The trialkyl silyl styrenes can b prepared by the e The hydrocarbon polymers from which the polymer reaction of nuclear trialkylsilyl phenyl magnesium halcarbon is made have been described above as formed ides with acetaldehyde to yield trialkyl silylated phenyl from hydrocarbon monomers. The hydrocarbon polya-methyl carbinols which are subsequently catalytically mers can also be formed from linear or network polydehydrated over activated alumina at a temperature of mers which contain only carbon atoms in the linear chains about 300 C to forni the trialkyl silyl styrenes, as de oi networks themselves but whic also contain substituent scribed more fully in the copending application of F H atoms or radicals containing elements other t an carbon Winslow Serial o 22 ,640, n e same day as and ydrogen such as oxygen, nitrogen, sulfur or halothe present application now Patent No 2 642 415. gens, and which upon heating are Converted to cross-linked 5U The presence of silicon in the polymer carbon results ydrocarbon olymers bus, polyvinyl alcohol which in an increase of electrical resistivity without any loss of is an essentiaily linear polymer, evolves its oxygen in modu ation us, apo ymer carbon formed by dehydro the form of water when heated to a temperature of 25.0 genating at 960 C a polymer of a mixture of tive parts C. in a non-oxidizing atmosphere The unsaturation inr by weight of divinyl benzene and four parts by weight roflueed y ille Splitting olf of ille SubSfifuenS reSulS "5 of ethyl vinyl benzene had an electrical resistivity of in extensive cross-linking so that, by the time substan- 1.5)(10`2 ohm-centimeters. When l0 per cent by weight tially all of the oxygen has been driven Oli, aS fOr 1n` of the monomer mixture of which the polymer was formed stance after about fifteen hours at 250 C., a hydrO- was replaced with trimethyl silyl styrene, the product of carbon polymer possessing adequate cross-linking for use dehydrogenation at 960 C. had a silicon content of 1.2 in the process of the present invention has been produced. per cent and had an electrical resistivity of 1X l0-l ohm- Similarly, polyvinylidene chloride and polyvinyl chlocentimeters. Larger amounts of silicon give progressively ride, both essentially linear polymers, evolve HClwhen higher resisflvities, heat@ 1n inert 0r non-OXldlZing atmospheres 0F 1n the The polymers containing normally solid elements other presence of dehalogenating agents and yield cross-linked than carbon can be Converted to modified polymer Carbon hydrocarbon Polymers Smtable for the Purposes of the by the same pyrolytic procedures as described above for heating in a non-oxidizing atmosphere are the polymers of vinyl acrylic acid, chlorovinyl acrylic acid, propenyl ethinyl carbinol, propenyl ethinyl ketone, vinyl ethinyl ketone, hex-3en-5yn-2ol and hex-3en-5yn2one- Certain of the polymers described above which are 7 convertible to cross-linked hydrocarbon polymers upon 5 Polymers Which Contain oxygen linkages 1n The actual thermal dehydrogenation of similarly shaped bodies of a network of the polymer itself, such as regenerated cellur0s$ liuked hydrocarbon polymerlose, phenolic resins 0r Polyester resms- The Oxygen in 2. A microphone as described in claim 1 wherein the the network appears to have a graphitizing influence since Carbon particles are discrete spheres,

Ille Carbon Produced fi'oin PolymefS Containing Such 3. A microphone as described in claim l wherein the oxygen is a different form of carbon which is substancarbon particles are made up of a mixture of discrete tially more graphitic and is softer. spheres and agglomerated spheres.

4. A microphone as described in claim 1 wherein the carbon particles are agglomerated spheres.

5. A microphone as described in claim 1 wherein the hydrocarbon polymer is a polymer of divinyl benzene.

6. A microphone as described in claim 1' wherein the hydrocarbon polymer is a copolymer of divinyl benzene and ethyl vinyl benzene.

7. A microphone as described in claim 1 wherein the hydrocarbon polymer is trivinyl benzene.

8. A microphone as describe in claim l wherein the modulation chamber is hermetically sealed and the free space therein is lled with an inert gas.

9. A microphone as described in claim 1 wherein the modulation chamber is hermetically sealed and the free 10. A carbon granule microphone having a modulaof hard, lustrous carbon formed by the thermal dehydrogenation of cross-linked hydrocarbon polymer particles and having outer coatings of carbon deposited pyrolytically from a hydrocarbon in the gaseous state.

11. A carbon granule microphone having a modulation chamber containing particles the surfaces of which are made u o hard, by the thermal dehydrogenation of a cross-linked hydrocarbonk 2 polymer.

12. A carbon granule microphone having a modulation chamber containing a microphonic material consistdehydrogenatron of similarly shaped parformed of carbon, hydro ing carbon containing normally solid by the thermal ticles of a cross-linked polymer gen and said additional element.

13. A carbon granule microphone as defined in claim 12 wherein said additional is silicon.

14. A carbon granule m'crophone as defined in claim 12 wherein said additional element is boron.

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1. A CARBON GRANULE MICROPHONE HAVING A MODULATION CHAMBER CONTAINING A MICROPHONIC MATERIAL CONSISTING OF HARD, LUSTROUS, COHERENT CARBON PARTICLES, HAVING SUBSTANTIAL AREAS OF SMOOTH, SPHERICAL SURFACE PRODUCED BY THE THERMAL DEHYDROGENATION OF SIMILARLY SHAPED BODIES OF A CROSS-LINKED HYDROCARBON POLYMER. 